Assuming that Noah uses a certain amount of sparkling water to make a liter of sparkling orange juice, let's call this amount "x" liters of sparkling water per liter of orange juice. Then, the total volume of sparkling orange juice that Noah can make from j liters of orange juice is given by:
t = x*j
Therefore, the equation that Noah can use to find t if he knows j is:
t = x*j